It is possible for a general sphygmomanometer to obtain a precise blood pressure value when blood pressure measurement is performed under a condition where a cuff is raised and the altitude thereof becomes the same as the altitude of a heart. However, in the case of an electronic sphygmomanometer applied to a wrist, an electronic sphygmomanometer applied to a finger, or the like, the sphygmomanometer is used under a condition where a cuff (integrated with a main body in many cases) is placed on the wrist, finger, or another predetermined region. The wrist and finger are regions that are capable of freely moving, so that it is impossible to set these parts so that the altitudes thereof become the same as the altitude of a heart. Therefore, there may be cases where it becomes impossible to precisely measure blood pressures.
To avoid such a problem, with a conventional technique, a uniaxial angle detecting apparatus is provided for a cuff or in the vicinity thereof, the positional relation between the cuff and a heart is determined from an angle detected by the apparatus, and there is notified a result of the determination.
However, if the positional relation between the heart and the cuff is determined only from uniaxial angle information like in the case of the conventional technique described above, various error causing factors affect the accuracy of the blood pressure measurement.
As such error causing factors, there may be first cited variations in upper arm length and forearm length among subject persons. For instance, FIG. 32 shows a case where measurement is performed for different forearm lengths L1 and L1a. Even if the same inclination angle of a forearm is maintained, there occurs a difference in altitude between a cuff 100 and a heart 101. That is, in the case where the forearm length is L1, the altitude of the cuff 100 becomes the same as the altitude of the heart 101. However, in the case where the forearm length is L1a, the actual position of a cuff 100a becomes lower than the position of the heart 101 by ΔH. Accordingly, if it is determined that this detected inclination angle is correct, there may be a case where it becomes impossible to precisely measure a blood pressure due to the variation in forearm length. As shown in FIG. 33, in case where the forearm length L1 and the inclination angle are not changed and the upper arm lengths L2 and L2a are different, for the upper arm length L2, the altitude of the cuff 100 becomes the same as the altitude of the heart 101, on the other hand, for the upper arm length L2a, the position of the cuff 100a becomes higher than the position of the heart 101 by ΔH. Accordingly, if it is determined that this inclination angle is correct, there may be a case where it becomes impossible to precisely measure a blood pressure due to the variation in upper arm length.
Also, even if the same angle of a forearm is maintained, there may be a case where the altitude of a heart differs from the altitude of the center of a cuff depending on the angle of an upper arm of a subject person, which is to say the position of his/her elbow, which causes an error. FIGS. 34 and 35 show examples of such a case. In FIG. 34A, the altitude of the cuff 100 is the same as the altitude of the heart H and the difference ΔH becomes zero under a condition where an upper arm L2 extends vertically. Even if the detected angle of a forearm is the same as that in FIG. 34A, under a condition where the upper arm L2 is tilted in the left-and-right direction (pitch direction) in front of a body as shown in FIG. 34B, the center of the cuff 100 is positioned higher than the heart H by ΔH. In a like manner, under a condition where the upper arm L2 is tilted in a back-and-forth direction (roll direction) of the body as shown in FIG. 35, the center of the cuff 100 is also positioned higher than the heart H by ΔH. Accordingly, if it is determined that the altitude of the cuff is the same as the altitude of the heart in the cases shown in FIGS. 34B and 35 because the inclination angle of the forearm L1 is the same as that shown in FIG. 34A, there occurs a problem in that erroneous results are obtained.
Also, in the case where a space is generated between an elbow and a body due to the movement of an upper arm and in the case where the body is inclined, even if the same angle of a forearm is maintained in the pitch direction, there may occur a difference between the altitudes of the heart H and the center of the cuff 100. There may be the cases where this causes an error. For example, in FIG. 36 there are shown a case where the body is inclined by 0° as indicated by a dotted line and a case where the body is inclined by 30° as indicated by a solid line. If the body is tilted while maintaining the same posture, the angle of the forearm with respect to a horizontal plane becomes small, so that the position of a sphygmomanometer is displaced from the position of the heart when the subject person tries to position the sphygmomanometer within a predetermined range. Also, as indicated by the solid line in FIG. 37, if the forearm is tilted with respect to a vertical direction, the position of the sphygmomanometer is displaced from the position of the heart like in the case shown in FIG. 36.
The present invention has been made to solve these problems of the conventional technique, and is aimed at providing an electronic sphygmomanometer that is capable of precisely measuring a blood pressure by more precisely determining the positions of a cuff and a heart.